In this study, the effects of various parameters on pressure drop reduction caused by adding small amounts of drag reducing polymers has been investigated in crude oil pipelines. In order to make a comprehensive analysis of various operating parameters such as temperature, oil flow rate, pipe diameter, pipe roughness, type of drag reducing agent (DRA), and concentration of DRA, some experiments have been carried out with several concentrations of three different DRAs in four different operating temperatures. The obtained results indicate that the amount of drag reduction increases with temperature, oil flow rate, pipe roughness as well as DRAs concentration. The DRA1 causes the highest drag reduction. Also, the experiments showed that DR% increases with decreasing pipe diameter. Hence, the ability of DRA increases with the relative roughness of pipes. (C) 2012 Elsevier B.V. All rights reserved.